Thin film resonator, or TFR, technology has received much interest over the last several years. The thin film resonator technology makes possible a class of thin film microwave acoustic devices that are truly compatible with active semiconductor circuitry. The small size of the thin film resonator is compatible with semiconductor technology, and the thin film resonator can be integrated with semiconductor devices onto a common substrate.
To operate in the fundamental mode at VHF to microwave frequency ranges, a resonator must have a thickness in the range of tens of microns to less than one micron. Devices of such thicknesses are very fragile and easily damaged, and require some form of external support during and after manufacture for any practical application. This requirement has given rise to the development of etching techniques which provide for the placing of the device on a silicon substrate, with a cavity etched into the silicon underneath the device to allow free movement of the device. This permits the edges of the device to be supported by the silicon substrate.
The basic thin film resonator technology uses DC magnetron sputtered highly-oriented thin films of dielectric material, preferably aluminum nitride (AlN) or zinc oxide (ZnO). The dielectric film is sandwiched between a pair of conductive electrodes, typically thin film aluminum electrodes, and the electrodes serve not only as electrical interconnections, but also acoustic reflecting surfaces for guiding and trapping the acoustic energy in the dielectric thin film. The acoustic cavity for the resonator is defined by the aluminum-silicon composite membrane structure. That membrane should be of low mass for high frequency operation, and that, in turn usually requires the removal of substrate material underlying the membrane portion of the thin film resonator.
In order to provide quality thin film resonators, a dielectric material must be used that has strong piezoelectric properties that make it useful in building thin film resonators. Aluminum nitride, for example, is a hard dielectric material which exhibits such piezoelectric properties, with a bandgap of 6.+-.0.5 eV. To be useful as a resonator at a given frequency, this material needs to have a corresponding thickness according to: EQU f=V.sub.1 /2T
where "V.sub.1 " is the longitudinal velocity of sound in the dielectric material of interest, "T" is the thickness and "f" is the desired frequency. For a V.sub.1 =11.times.10.sup.5 cm/sec along the c-axis, a 1 GHz response requires about 5 microns of aluminum nitride. At optimum parameters, this thickness requires about 90 minutes of deposition, a rather long interval.
In order to achieve the desired piezoelectric properties, the crystal grain growth of the thin films must be oriented. This orientation is controlled in the DC magnetron sputtering system with the electrostatic fields involved, the magnetic fields of the magnets, the bipolar field of the ground shield and target, and the field from the anode ring.
However, when using relatively long deposition times with a dielectric material like aluminum nitride, as aluminum nitride is deposited on the inner surfaces of the chamber of the sputtering system, the chamber becomes electrically insulated as the deposition proceeds. This in turn causes instability in the plasma and changes in the electrostatic fields. As the dielectric buildup increases, impurities from the chamber are more likely to be dislodged by stray ions and contaminate the dielectric thin films. This contamination significantly changes the uniformity and orientation of the thin films to a less desirable state, thereby decreasing the piezoelectric response of the dielectric thin films. In fact, it has been determined that these detrimental changes can significantly affect the quality of the dielectric thin films in as little as 45 minutes of deposition.